The present invention relates to an improved composition for removing both H2S and mercaptans from a gas stream using an aqueous scrubbing solution.
Gases containing acid gases, such as one or more of CO2, COS and H2S, such as gas streams from natural gas wells, also in many instances contain undesirable quantities of mercaptans. Solvents known in the art as “physical solvents” and “chemical solvents” and/or mixtures thereof are useful for the absorption of acid gases from these types of gas streams. This process is often referred to as scrubbing or washing of the gas stream. When using a chemical solvent typically an acid gas rich gas stream is contacted (scrubbed) with an aqueous composition comprising an amine solution at low temperature or high pressure in an absorber to produce a gas stream containing a lesser amount of the acid gases and an acid gas enriched aqueous solution. The aqueous amine solution usually contains an alkanolamine such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA) hydroxyaminoethyl ether or diglycol amine (DGA). Typical physical solvents include cyclotetramethylenesulfone (sulfolane) and its derivatives, aliphatic acid amides, NMP (n-methylpyrrolidone), N-alkylated pyrrolidones and corresponding piperidones, methanol and mixtures of dialkylethers of polyethylene glycols. An aqueous mixture of a chemical solvent, for example DIPA and/or MDEA and a physical solvent, for example sulfolane, can also be used. A physical solvent employs nonreactive interactions to dissolve the acid gas while the chemical solvent adds a reaction to enhance the degree of acid gas removal. A sufficient amount of the acid gases are removed from the gas stream to render the gas suitable for sale or additional downstream conditioning. The acid gas enriched solution is regenerated for reuse by driving off the absorbed acid gases in specialized equipment such as flash tanks and/or stripper columns typically operated at high temperature or low pressure and the chemical or physical solvent is reused in the process. In the past it has, however, been difficult to remove mercaptans from gas streams during the absorption step because even though the mercaptan is an acid gas it is a weaker acid than either H2S or CO2 and thus is not normally removed along with these other acid gases.
Mercaptans are substituted forms of H2S in which a hydrocarbyl moiety, R, takes the place of one of the hydrogen atoms. Their general formula is RSH. The properties of mercaptans depend substantially on the length of the hydrocarbon chain. Mercaptans in aqueous solution likewise act as acids, but are significantly weaker than H2S. With increasing length of the hydrocarbon chain, therefore, mercaptans behave like hydrocarbons, which make their removal from hydrocarbon gas streams particularly difficult because the physical or chemical solvent prefers to take out the H2S and or CO2. Mercaptans occur in some natural gas sources, especially on the North American continent, and are typically present in most liquid or liquefied refined hydrocarbon products (LPG). However, because of their corrosive and malodorous properties, it is usually preferred to remove mercaptans from hydrocarbon gases. Treated and purified hydrocarbons for polymerization reactions, for example, should customarily contain not more than 1-20 ppm of mercaptans.
It is known that certain compounds, such as piperazine and monomethylethanolamine (MMEA) may be advantageously added to aqueous amine solutions and mixtures of chemical and physical solvents to enhance their absorption capabilities. For example, U.S. Pat. No. 4,336,233 discloses a process for removing CO2, H2S and in some cases COS from gases that contain these compounds by washing the gases with aqueous solutions containing MDEA and piperazine as an absorption promoter. The amount of piperazine stated to be useful in these absorption compositions is up to 0.8 moles per liter of aqueous solution. The '233 patent also discloses that it is particularly preferred to add from 0.05 to 0.4, especially from 0.2 to 0.4, moles piperazine per liter of aqueous solution. However, the addition of these materials is not disclosed as improving the removal of mercaptans.
U.S. Pat. No. 4,997,630 discloses a staged process for the removal of CO2 and/or H2S using an aqueous absorption liquid containing MDEA. The aqueous absorption liquid may additionally contain from 0.05 to 1, preferably 0.1 to 0.8, mole per liter piperazine.
U.S. Pat. No. 6,337,059 teaches that an improved absorption compositions for the removal of acid gases from gas streams that comprise aqueous MDEA/piperazine solutions containing greater than 1 mole piperazine per liter of aqueous solution. In particular, aqueous solutions containing greater than 1 mole piperazine per liter of aqueous solution and about 1.5 to about 6 moles MDEA per liter of aqueous solution were found to provide superior absorption capabilities over aqueous MDEA/piperazine solutions containing 1 mole or less piperazine per liter of aqueous solution. It also teaches that MDEA/piperazine absorption compositions advantageously provide equivalent CO2 removal at reduced absorption composition circulation rates in the absorber and increased absorber temperature, as well as improved performance of downstream regeneration equipment. However, none of these piperazine processes effectively remove mercaptans from the gas.
U.S. Patent Application Publication 2005/0166756 discloses a process for the removal of carbon dioxide from a gas stream containing carbon dioxide by washing the gas with an aqueous washing solution containing between 15 and 45 parts of water by weight, based on total solution weight, between 15 and 40 parts of sulfolane, and between 30 and 60 parts of a secondary or tertiary amine derived from ethanol amine, wherein the amounts of water, sulfolane and amine together equal 100 parts by weight, the process being carried out in the presence of a primary or secondary amine compound in an amount between 0.5 and 15 wt % based on water, sulfolane and amine.
U.S. Pat. No. 6,740,230 teaches a process for removing mercaptans from a fluid stream by contacting the fluid stream in an absorption or extraction zone with a scrubbing liquor having 1) at least one tertiary aliphatic alkanolamine of 3-12 carbon atoms, and 2) from 0.5 to 15% by weight of at least one activator selected from the group consisting of saturated 5-membered heterocyclic, saturated 6-membered heterocyclic, and a mixture thereof.
WO 2004/047955 teaches a process for the removal of mercaptans from a gas stream by using an aqueous absorption composition having 10 to 40 wt % of a physical solvent and 20 to 60 wt. % of an amine. The physical solvent utilized is sulfolane. After the absorption step, the treated gas runs through a molecular sieve for further removal of mercaptans.
Cyclodextrins are known to absorb (form inclusion complexes with) organic molecules or portions of organic molecules and have been used in aqueous solutions to absorb mercaptans from surfaces, see U.S. Pat. No. 5,534,165, and also to function in hair waving solutions to remove the mercaptan smell, see U.S. Pat. No. 4,548,811.
Despite the existence of various absorption compositions for mercaptan removal, there still exists a need for an improved composition and method that can have a better absorption capacity for mercaptans from hydrocarbon gases that also contain stronger acid gases such as H2S and CO2.